Seamless tubes intended for heat exchangers or for steam generators undergo straightening and bending operations before assembly.
In particular, tubes made of the nickel alloy Inconel 600, used in the steam generators of nuclear power stations, undergo these operations at the end of the manufacturing cycle, just before the hydrostatic test during which water under pressure is fed into the tube, this test being intended to separate the tubes which fail.
The final operations of straightening and bending these tubes introduce longitudinal and circumferential residual stresses into the tubes. Where these residual stresses are tensile stresses, they are superimposed on the stresses applied to the tubes in operation (compressive stresses, bending stresses and thermal stresses.
The presence of these residual stresses, superimposed on the stresses which the tube undergoes in operation, is detrimental to the corrosion stress cracking resistance of these tubes. In fact, as regards corrosion stress cracking, it is the highest total principal stresses, whether longitudinal or circumferential, which determine the direction of cracking.
It is thus necessary to remove, from the tubes of steam generators, the residual stresses originating from the mechanical treatments which these tubes have undergone at the end of the manufacturing cycle.
Thermal stress release processes for removing these residual stresses have been proposed, which consist of keeping the tubes at a temperature of about 700.degree. C. for a defined time. The operation of these heat treatments however requires the construction of very large ovens into which the tubes, which are themselves of large size, can be introduced, and this greatly increases the cost of the installations required for the manufacture of steam generator units for nuclear power stations.
On the other hand, mechanical stress release processes are known, which are applied to semi-finished goods such as bars or sheets to remove the residual stresses originating from the forming process. These mechanical treatments introduce permanent deformations of large amplitude into the semi-finished goods thus treated, and can therefore not be used in the case of finished articles such as steam generator tubes, where rigorously precise dimensions are required.
Mechanical stress release treatments applied to welded articles to remove the residual stresses in the vicinity of the zone affected by the welding process are also known. In particular, a process has been proposed for the stress release of very thick fabricated vessels by applying an internal hydrostatic pressure slightly greater than the test pressure of the vessels. Because the welded zone has a lower strength it is not possible to use a high internal pressure see U.S. Pat. No. 3,343,249 and "Reduction of Residual Sresses in Welded Joints . . . ;" Etingov and Mikhaila; Svar, 1972 No. 12 pp 38-40; consequently the residual stresses remain at a high level after the internal pressure treatment, so that this treatment is of very mediocre efficiency.
In the case of Inconel tubes for steam generators of nuclear power stations, the level of residual stresses must be sufficiently low to avoid the corrosion stress cracking phenomenon when these tubes are put into service, and it is insufficient to put these tubes under hydrostatic pressure if the pressure remains near the test pressure of the tube.